how to take value from Label or Textbox using Javascript ?

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ASP.NET

Javascript

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I ma using a javascript function to convert input Text of textbox in some other languges like HINDI,MARATHI etc.
but this function accepts only one languge statically.
so i want to give language code like "HINDI","MARATHI" from some other textbox value, but i cant get this language code from textbox and hence function is not working ,,so please do suggest proper way to give LanguagCode to the Function in javascript
Here is my code:

<script language="javascript" type="text/javascript">
    google.load("elements", "1", {
        packages: "transliteration"
    });

    function onLoad() {
        
       value1 = document.getElementById('<%=txtLang.ClientID%>').value;
       
       var gt= $("#lblLang").text();
        //var gt = HINDI;
        //alert('language :', lang);
        var options = {

            sourceLanguage: google.elements.transliteration.LanguageCode.ENGLISH,
            destinationLanguage: google.elements.transliteration.LanguageCode.valueOf(value1), // available option English, Bengali, Marathi, Malayalam etc.
            shortcutKey: 'ctrl+g',
            transliterationEnabled: true
        };

        var control = new google.elements.transliteration.TransliterationControl(options);
        control.makeTransliteratable(['txtHindiContent']);
    }
    google.setOnLoadCallback(onLoad);

</script>

Design SIde:

<table>
    <tr>
        <td>Type in Hindi : </td>
        <td>
            <asp:TextBox ID="txtHindiContent" runat="server" ClientIDMode="Static" TextMode="MultiLine" Height="100" Width="250"></asp:TextBox>
        </td>
    </tr>
    <tr>
        
        <asp:TextBox ID="txtLang" runat="server" Text="GUJARATI" xmlns:asp="#unknown"></asp:TextBox>
        <span id="spn" runat="server" title="HINDI"></span>
            </td>
        <td><asp:Button ID="btnSave" runat="server" Text="Save" OnClick="btnSave_Click" /></td>
    </tr>